Isolation structure and fabricating method therefor

ABSTRACT

A semiconductor device isolation structure and a fabricating method therefor are disclosed. The isolation structure includes a trench which is formed on an isolating region to define an active region. First, second, and third insulating layers are deposited in the trench. The second insulating layer has an etch selection ratio different from those of the first and third insulating layers. The edge portions of the third insulating layer which contact the side walls of the trench characteristically do not show any collapse. Therefore, when supplying a subthreshold voltage, a hump phenomenon does not occur. As a result, leakage current is kept from increasing, and the device refresh characteristic can be kept from deteriorating. Further, the third insulating layer covers the top edge portions of the trench. Therefore, the gate insulating layer (which is formed later) has a sufficient thickness. Therefore, yield voltage characteristics can be kept from deteriorating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor isolation structure andfabricating method therefor. Particularly, the present invention relatesto a semiconductor isolation structure and a fabricating method in whichthe size of the active region is kept from decreasing because of anincrease in the size of the isolating region, by utilizing a trench.

2. Description of Related Art

As the density of semiconductor devices increases, reducing the size ofan isolating region in such semiconductor devices is more and moreimportant.

Generally, a semiconductor device is isolated by applying the LOCOS(local oxidation of silicon) method. In the LOCOS method, a pad oxidelayer is formed between a silicon nitride layer and a semiconductorsubstrate by thermal oxidation in order to relieve stress caused bydifferent rates of thermal expansion between the semiconductor deviceand the silicon nitride layer. The silicon nitride layer is formed onthe active region and is used as a hard mask. Further, a fieldinsulating layer defines a device isolating region which is formed byoxidizing the field region of the semiconductor substrate not covered bythe silicon nitride layer. The field insulating layer grows in thevertical direction and in the horizontal direction relative to thesubstrate because of an oxidant (O₂) diffusing along the pad oxidelayer. Therefore, the field insulating layer grows under the patternedge of the silicon nitride layer as well as on the field region.

The encroachment of the field insulating region into the active regionis called a “bird's beak”. The length of the bird's beak eventuallybecomes one half of the thickness of the field insulating region.Therefore, if the size of the active region is to be kept fromdecreasing, the bird's beak must be minimized.

In order to reduce the length of the bird's beak, a conventional methodis known in which the thickness of the field insulating region isdecreased. However, if the thickness of the field insulating region isreduced in a 16M DRAM or higher, then parasitic capacitance between thewiring layer and the semiconductor substrate increases. Therefore, thesignal transmitting speed is lowered. Further, parasitic transistors areformed in the isolating regions due to the wiring of the gate.Accordingly, the threshold voltage V_(t) is lowered, with the resultthat the device isolating characteristics deteriorate.

Therefore, conventional methods are known in which the length of thebird's beak is decreased, and at the same time, the device isolatingcharacteristics are improved. One of these methods is PBLOCOS (poly Sibuffered LOCOS), in which the thickness of the stress absorbing padoxide layer is reduced, and a poly crystalline silicon layer is insertedinto between the semiconductor substrate and the silicon nitride layer.Another is SILO (sealed interface LOCOS) in which the side walls of thepad oxide layer are protected by a silicon nitride layer. A third one isrecessed oxide LOCOS in which a recessed field insulating layer isformed on the field region of the semiconductor substrate.

However, in the above conventional methods, too precise a flatness andtoo precise a design rule are required to make these methods applicableto the 256M DRAM scale.

Therefore, in order to overcome the above problems, a BOX (buried oxide)type shallow trench isolation method is best known. In this method, ashallow trench is formed on the semiconductor substrate. Thereafter, aburied silicon oxide layer is deposited by a chemical vapor depositionmethod (CVD method). Therefore, the bird's beak is not formed, so theactive region is not encroached. Further, the silicon oxide layer isburied into the trench and subsequently etched back, thereby obtaining aflat surface.

FIG. 1 is a sectional view showing an isolation structure according tothe conventional art.

In this isolation structure, a field insulating layer 23 is formedwithin a trench 17 defining the isolating region of the semiconductorsubstrate 11. That is, CVD is used to deposit the field insulating layer23 into the trench 17. For this purpose, after depositing an insulatinglayer 23 composed of silicon oxide or the like using CVD, the insulatinglayer is etched back so that the insulating layer 23 remains only withinthe trench 17.

That is, the trench 17 is filled with the field insulating layer 23 byapplying a CVD process instead of an oxidation process. Therefore theformation of the bird's beak is prevented. Further, etching back isperformed so as to make the silicon oxide insulating layer 23 remainwithin the trench. A flat surface is therefore obtained.

FIGS. 2 to 5 illustrate a method for fabricating the above-describedconventional isolation structure.

Referring to FIG. 2, a pad oxide layer 13 is formed on semiconductorsubstrate 11 by a thermal oxidation process. A silicon nitride layer 15is formed on the pad oxide layer 13 by CVD. A photolithography processis then carried out to form a photolithographic pattern, so thatpredetermined portions of the pad oxide layer 13 and the silicon nitridelayer 15 are removed to thereby expose the substrate 11. This defines adevice isolating region I1 and an active region A1. By using the siliconnitride layer 15 as a mask, reactive ion etching (to be called “RIE”below) is carried out to form the trench 17.

Referring to FIG. 3, silicon oxide is deposited by CVD to fill thetrench 17, thereby forming an insulating layer 19. Here, because of theheight difference between the surface of the silicon nitride layer 15and the bottom of the trench 17, a step is formed on the surface of theinsulating layer 19.

A photoresist layer 21 is coated on the insulating layer 19. Thephotoresist layer 21 is patterned to expose locations corresponding tothe silicon nitride layer 15 and to remain over locations correspondingto the trench 17 on the insulating layer 19. Then by using the patternedphotoresist layer 21 as a mask, the exposed portions of the insulatinglayer 19 are etched. Here, the etched surface of the insulating layer 19is roughly as high as the top of the trench 17.

Then, as shown in FIG. 4, the patterned photoresist layer 21 is removed.Then the insulating layer 19 is partially removed to expose the siliconnitride layer 15 by, for example, chemical-mechanical polishing process(to be called “CMP process” below).

Then, as shown in FIG. 5, a wet etching process is carried out to etchthe pad oxide layer 13 and the silicon nitride layer 15 so as to exposethe substrate 11. Under this condition, the upper portion of theinsulating layer 19 is also removed so that it remains only within thetrench 17. Furthermore, its surface becomes even with the substrate 11,thereby ensuring a flat surface. Thus the insulating layer 19 whichremains within the trench 17 becomes a field insulating layer 23.

In this related art, when the pad oxide layer 13 is etched, the upperportion of the filled insulating layer 19 is also etched, and thus afield insulating layer 23 is formed. Under this condition, however, theinsulating layer 19 is isotropically etched. Therefore, the field oxidelayer is formed to collapse the field insulating layer at the portionwhich contacts the side wall of the trench. Due to the collapsed portionof the field insulating layer, when a subthreshold voltage is supplied,a hump phenomenon occurs. As a result, current leakage increases, andthe refresh characteristics deteriorate. Further, the electric field isconcentrated at the top edges of the trench. Therefore, the thresholdvoltage of the subsequently formed gate insulating layer becomes weak.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the above-describeddisadvantages of the related art.

It is an object of the present invention to provide an isolationstructure that avoids increased leakage currents and deterioration inrefresh characteristics at a portion of the field insulating layercontacting the side wall of the trench.

It is another object of the present invention to provide an isolationstructure that avoids deterioration of the threshold voltagecharacteristic of the gate insulating layer.

It is still another object of the present invention to provide afabricating method for an isolation structure that avoids collapse ofthe portion of the field insulating layer contacting the side wall ofthe trench.

In achieving the above objects, an isolation structure according to thepresent invention includes: a semiconductor substrate having a trenchdefining an isolating region and an active region of the semiconductorsubstrate; and first, second, and third insulating layers deposited inthe trench, the second insulating layer being composed of a materialwith an etch selection ratio different from those of the first and thirdinsulating layers.

In another aspect of the present invention, the fabricating method foran isolation structure according to the present invention includes:forming a masking layer on a semiconductor substrate to expose anisolated region; forming a trench in this exposed region; depositing afirst insulating layer within the trench, leaving a remaining portion ofthe trench unfilled; removing the masking layer; forming a secondinsulating layer over exposed side walls of the trench and the surfaceof the first insulating layer, where the second insulating layer iscomposed of a material with an etch selection ratio different from thatof the first insulating layer; forming a poly crystalline silicon layerupon the second insulating layer to fill the vacant portion of thetrench; oxidizing the poly crystalline silicon layer to form a thirdinsulating layer so as to make it project and so as to cover the topedges of the trench; and removing the exposed portions of the secondinsulating layer.

In still another aspect of the present invention, the fabricating methodaccording to the present invention includes: forming a masking layer ona semiconductor substrate to expose an isolating region; forming atrench in the region thus exposed; forming a first insulating layerwithin the trench, leaving a remaining portion of the trench unfilled;removing the masking layer; forming a second insulating layer over theexposed side walls of the trench and over the surface of the firstinsulating layer, where the second insulating layer is composed of amaterial with an etch selection ratio different from that of the firstinsulating layer; forming a silicon oxide layer upon the secondinsulating layer and over the substrate; and etching back the siliconoxide layer to form a third insulating layer, where the third insulatinglayer fills the remaining vacant portion of the trench, and where theedges of the second insulating layer are exposed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other advantages of the present invention willbecome more apparent by describing in detail the preferred embodimentsof the present invention with reference to the attached drawings inwhich:

FIG. 1 is a sectional view showing a conventional isolation structure;

FIGS. 2 to 5 illustrate a method for fabricating the conventionalisolation structure;

FIG. 6 is a sectional view showing an isolation structure according tothe present invention;

FIGS. 7 to 10 illustrate a method for fabricating the isolationstructure according to the present invention; and

FIGS. 11 to 12 illustrate a method for fabricating the isolationstructure according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 is a sectional view showing an isolation structure according tothe present invention.

In the isolation structure according to the present invention, a fieldinsulating layer 47 is formed by depositing insulating materials into atrench 37 defining an active region A2 of a semiconductor substrate 31.

The trench 37 is formed in the isolating region 12, for defining anactive region A2 of the substrate 31. This trench 37 is formed to adepth of 2500-5000 Å by an anisotropic etching such as an RIE process orthe like.

The field insulating layer 47 consists of first, second and thirdinsulating layers 39, 41 and 45 which are stacked in the trench 37. Thefirst and third insulating layers 39 and 45 are composed of siliconoxide, but the second insulating layer 41 is silicon nitride, which hasa different etch rate from that of the third insulating layer 45.

In the above, the first insulating layer 39 is deposited into the trench37 by the CVD process. Then an etch-back using a CMP or RIE process iscarried out so that the upper portion (500-1500 Å) of the firstinsulating layer 39 in the trench 37 is removed. Then the secondinsulating layer 41 is deposited in the trench 37 to a thickness ofabout 50-150 Å by the CVD process so as to cover the exposed side wallsof the trench 37 and the surface of the first insulating layer 39 andthe substrate 31. Then a third insulating layer 45 is formed, and thenthe exposed portions of the second layer 41 are etched away.

Specifically, the third insulating layer 45 is formed by oxidizing apoly crystalline silicon deposited in the remaining vacant portion ofthe trench 37, up to the surface of the substrate 31 and over the secondinsulating layer 41. The third insulating layer 45 is completed byoxidizing the poly crystalline silicon. When poly crystalline silicon isoxidized, its volume expands by about 60-70%. Therefore, the thirdinsulating layer 45 expands vertically to project 300-1000 Å above thesubstrate 31 to form a step. Third insulating layer 45 also horizontallyexpands to cover the edges of the top of the trench 37.

Therefore, the portion of the third insulating layer 45 which contactsthe tope edge of the trench 37 does not collapse. When a subthresholdvoltage is applied, the hump phenomenon therefore does not occur.Consequently, leakage current does not increase, and the refreshcharacteristic is not negatively affected. Further, the third insulatinglayer 45 covers the top edges of the trench 37. Therefore, a gateinsulating layer (not shown) which is formed later is not so thin thatits threshold voltage characteristic is not weakened. During the thermaloxidation for forming the third insulating layer 45, the second filledinsulating layer 41 prevents the substrate 31 from being oxidized.

As discussed above, the third insulating layer 45 is formed by thermallyoxidizing the poly crystalline silicon layer deposited into theremaining vacant portion of the trench 37. In another embodiment of thepresent invention, silicon oxide is deposited upon the second insulatinglayer 41, and an etch-back is carried out until the second filledinsulating layer 41 is exposed.

FIGS. 7 to 10 illustrate a method for fabricating an isolation structureaccording to the present invention.

Referring to FIG. 7, a pad oxide layer 33 is formed to a thickness ofabout 100-200 Å upon a semiconductor substrate 31 by a thermal oxidationprocess. A silicon nitride layer 35 is formed to a thickness of about1000-2000 Å upon the pad oxide layer 35 by the CVD.

Then a photolithography process is carried out to pattern apredetermined portion of the pad oxide layer 33 and the silicon nitridelayer 35 until the substrate 31 is exposed. Thus a device isolatingregion 12 and an active region A2 are defined. Using the silicon nitridelayer 35 as a mask, the exposed portions of the substrate 31 are etchedby a reactive ion etching (RIE) process using an etching gas, forexample, including chlorine, thereby forming a trench 37.

Referring to FIG. 8, a first insulating layer 39 is deposited within thetrench 37. The first insulating layer 39 is formed by the CVD processupon the silicon nitride layer 35, in such a manner that silicon oxidepartially fills the trench 37, leaving the upper 500-1500 Å of thetrench vacant. Accordingly, a portion of the side walls of the trench 37not covered by the first insulating layer 39 remains exposed.

Referring to FIG. 9, the pad oxide layer 33 and the silicon nitridelayer 35 are removed by using, for example, a fluoric acid solution (HF)and a phosphoric acid solution (H₂PO₄). Then silicon nitride isdeposited to a thickness of about 50-150 Å by a CVD process to cover theexposed side walls of the trench 37 and the surface of the firstinsulating layer 39, thereby forming a second insulating layer 41.

Then a poly crystalline silicon layer 43 is deposited into the remainingvacant portion of the trench 37 over the second layer 41. The CVDprocess is used in depositing the poly crystalline silicon intoremaining vacant portions of the trench 37, thereby forming a polycrystalline silicon layer 43. Then an etch-back process such as CMP orRIE is carried out to planarize the surface of the structure, so thatthe poly crystalline silicon layer 43 remains only within the trench 37,and so that the second insulating layer 41 is exposed. Therefore, thepoly crystalline silicon layer 43 has a thickness of about 500-1500 Å.The second insulating layer 41 characteristically has a different etchrate from that of silicon. It is therefore utilized as an etch stoplayer to prevent the substrate 31 from being damaged.

Referring to FIG. 10, the poly crystalline silicon layer 43 is thermallyoxidized to form a third insulating layer 45. When the poly crystallinesilicon layer is oxidized, its volume expands by about 60-70%. The thirdinsulating layer 45 therefore not only vertically expands to form a stepabout 300-1000 Å high relative to the surface of the substrate, but alsohorizontally expands to cover the top edges of the trench 37. Therefore,the edge portions of the third insulating layer 45 do not collapse. Whenoxidizing the poly crystalline silicon layer 43, the second insulatinglayer 41, acting as an etch stop layer, acts to prevent the substrate 31from being oxidized.

The second insulating layer 41 is then selectively etched by using, forexample, a phosphoric acid solution (H₂PO₄) to expose the portions ofthe substrate 31. Because the third insulating layer 45 has a differentetch rate from that of the second insulating layer 41, and the thirdinsulating layer 45 is not etched, the collapse is avoided.

The above-described first, second and third insulating layers 39, 41 and45 collectively form a field insulating layer 47.

FIGS. 11 and 12 illustrate a method for fabricating an isolationstructure according to another embodiment of the present invention. Inthis embodiment, elements having the same constitutions and functions asthose of the first embodiment will be assigned with the same referencenumbers.

Referring to FIG. 11, after carrying out the step illustrated in FIG. 8,the pad oxide layer 33 and the silicon nitride layer 35 are respectivelyremoved by using, for example, a fluoric acid solution (HF) and aphosphoric acid solution (H₂PO₄).

A CVD process is then carried out to deposit a silicon nitride layerwith a thickness of 50-150 Å so that the exposed side walls of thetrench 37 and the surface of the first insulating layer 39 are covered,thereby forming a second insulating layer 41. Then a silicon oxide layeris deposited upon the second layer 41 by, for example, a CVD process toform a silicon oxide layer 49.

Referring to FIG. 12, the silicon oxide layer 49 is etched back by, forexample, a CMP or RIE process until the second insulating layer 41 isexposed, thereby forming a third insulating layer 51. The secondinsulating layer 41 has a different etch rate from that of the siliconoxide of the third insulating layer 51. Therefore, it functions as anetch stop layer so that damage to the substrate 31 is prevented.Further, the surface of the third insulating layer 51 is planarized byetching back the silicon oxide layer 49.

The second insulating layer 41 is selectively etched by using, forexample, a phosphoric acid solution or the like until the substrate 31is exposed. Under this condition, the third insulating layer 51 has anetch selection ratio different from that of the second insulating layer41. Therefore, its edge portions do not collapse.

The above-described first, second and third insulating layers 39, 41 and51 collectively form a field insulating layer 53.

According to the present invention as described above, the edge portionsof the third insulating layer 51 (where they contact the side walls ofthe trench 37) do not show any collapse. Therefore, when a subthresholdvoltage is applied, the hump phenomenon does not occur. As a result, theleakage current does not increase, and the deterioration of the refreshcharacteristic can be prevented. Further, the third insulating layer 51covers the top edges of the trench 37. Therefore, the later-formed gateinsulating layer has a sufficient thickness to prevent deterioration ofyield voltage characteristics.

What is claimed is:
 1. A method for fabricating an isolation structurefor a semiconductor device, comprising: forming a trench in asemiconductor substrate; depositing a first insulating layer in thetrench so as to partially fill the trench; depositing a secondinsulating layer over exposed side walls of said trench and the firstinsulating layer deposited in the trench, wherein the second insulatinglayer has an etch selection ratio different from that of the firstinsulating layer; and forming a third insulating layer in a remainingportion of the trench over a portion of the second insulating layerdeposited in the trench, wherein said third insulating layer projectsabove the surface of the semiconductor substrate and over respectiveedges of the trench.
 2. The method according to claim 1, wherein formingthe trench comprises: forming a pad oxide layer over the semiconductorsubstrate; forming a silicon nitride layer over the pad oxide layer;selectively etching portions of the pad oxide layer and the siliconnitride layer formed thereover so as to expose a portion of thesemiconductor substrate; and etching the exposed portion of thesemiconductor substrate so as to form the trench.
 3. The methodaccording to claim 2, wherein etching the exposed portion of thesemiconductor substrate comprises reactive ion etching.
 4. The methodaccording to claim 2, wherein depositing the first insulating layer inthe trench so as to partially fill the trench comprises depositing asilicon oxide.
 5. The method according to claim 4, wherein depositing asilicon oxide comprises depositing a silicon oxide using chemical vapordeposition.
 6. The method according to claim 1, wherein depositing asecond insulating layer comprises depositing silicon nitride over theexposed sidewalls of the trench, a surface of the semiconductorsubstrate, and the first insulating layer.
 7. The method according toclaim 6, wherein the second insulating layer is 50-150 angstroms.
 8. Themethod of claim 6, wherein forming a third insulating layer comprisesdepositing a polycrystalline silicon layer in the trench over thesilicon nitride layer, etching back the polycrystalline silicon layer soas to expose the silicon nitride layer formed over the surface of thesemiconductor substrate, and thermally oxidizing the etchedpolycrystalline silicon layer.
 9. The method of claim 8, furthercomprising wet etching an exposed portion of silicon nitride layercovering the surface of the semiconductor substrate so as to expose thesemiconductor substrate.
 10. The method of claim 6, wherein forming athird insulating layer comprises depositing a silicon oxide layer overthe silicon nitride layer, etching back the silicon oxide layer untilthe silicon nitride layer formed over the surface of the semiconductorsubstrate is exposed, etching back an exposed portion of the siliconnitride layer.
 11. The method according to claim 1, wherein the step ofdepositing the first insulating layer in the trench includes leaving aportion of the trench sidewall exposed.
 12. The method of according toclaim 1, wherein the step of depositing a second insulating layer overthe exposed sidewalls includes depositing the second insulating layerdirectly on the semiconductor substrate of the trench sidewalls.